Material properties and processes can appear remarkably different when viewed at the nanometer level. Mechanically, solids can approach the behavior of perfect single-crystals and inter-particle interactions become dominated by surface energetics and dyna mics. In thin films with nanometer scale grains, dislocation-loop formation can become unfavorable and grain sliding can become the dominant mechanism for plastic deformation. The study of the mechanical properties of these materials is presently enjoying increasing attention due in large part to the rapid development of scanning-probe techniques capable of making measurements on individual grains down to the nanometer level. In this presentation, I will illustrate some of these unique nanoscale effects in various applications of the Interfacial Force Microscope (IFM) to studies of the nanomechanical properties of thin films. The IFM is a scanning force-probe microscopy similar to the Atomic Force Microscope but distinguished by its use of a mechanically stable, zero compliance force sensor. Used in a nanoindenter mode, this sensor offers accurate control of the probe-sample separation and provides a quantitative measure of the film's mechanical behavior. I will illustrate the IFM's unique capabilities w i th examples of near theoretical yield strength for single-crystal surfaces and the effect of surface steps on the strength. In polycrystalline films, I contrast the mechanical behavior as a function of grain size and film thickness and discuss the mechanisms responsible for the interesting observations. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-94AL85000.